Method and apparatus for the control of a spraying apparatus

ABSTRACT

A method and apparatus controls intake air humidification spraying with at least two spraying nozzles ( 1   a   , 1   b   , 1   c   , 1   d ) for spraying a liquid into the intake air. When an amount of the liquid to be supplied through the spraying nozzles increases, liquid flow passages are opened for more nozzles ( 1   a,    1   b,    1   c,    1   d ) and/or the liquid flow is directed into a flow passage leading to one of the spraying nozzles that permits a larger liquid flow per unit of time than another of the spraying nozzles, and when an amount of the liquid to be supplied through the nozzles decreases, liquid flow channels are closed at least for one of the spraying nozzles ( 1   a,    1   b,    1   c,    1   d ) and/or the liquid flow is directed to one of the spraying nozzles that permits a smaller liquid flow per unit of time than another of the spraying nozzles.

BACKGROUND OF THE INVENTION

The present invention relates to a method as defined in the preamble ofclaim 1 for controlling a spraying apparatus, especially A sprayingapparatus designed for the humidification of the intake air of a pistonengine.

The invention also relates to an apparatus as defined in the preamble ofclaim 14.

The exhaust gases of piston engines, in particular diesel engines,contain many kinds of noxious combustion products. At the highcombustion temperatures, the combustion process in the cylinders of apiston engine produces nitrogen oxides (NOx), which are emitted togetherwith the exhaust gases into the atmosphere. Because of the harmfulenvironmental effects of nitrogen oxide emissions, efforts areundertaken to minimize their production.

As is known, adding water to the combustion process reduces thegeneration of nitrogen oxides. This phenomenon is based on the coolingeffect of water. In practice, the introduction of water into thecombustion process is often implemented by injecting water into theintake air. These arrangements are advantageous in respect of efficiencyof the engine. The maximum amount of water introduced into thecombustion space of the engine may advantageously be that amount whichwill remain in gaseous form in the intake air pressure and temperatureconditions.

The object of the invention is to achieve a completely new type ofcontrol system that allows the intake air to be humidified in a desiredmanner in different engine load conditions.

Another object of the invention is to achieve a control apparatus thatallows the humidification of intake air in different engine loadconditions.

The method of the invention is characterized in that, in accordance withthe control system's instructions, when the required amount of water tobe supplied through the nozzles increases, liquid flow passages areopened for more nozzles and/or the liquid flow is directed to a nozzlethat permits a larger liquid flow through it per unit of time, and whenthe required amount of water to be supplied through the nozzlesdecreases, liquid flow passages are closed at least for some of thenozzles and/or the liquid flow is directed to a nozzle that permits asmaller liquid flow through it per unit of time.

The method of the invention is additionally characterized by what isstated in claims 2-13.

The apparatus of the invention is characterized by what is stated inclaims 14-25.

The solution of the invention has numerous significant advantages. Byapplying the method of the invention, a very accurate control of aspraying apparatus can be easily achieved using a relatively economicalpump unit. By providing the spraying head with nozzles having differentproperties, the amount and/or characteristics of the water mist to beinjected can be varied in a desired manner by opening and closing themedium channels leading to different nozzles in the spraying head. Byproviding a constant-output pump and a return line whose k-value alwayscorresponds to the k-values of the closed nozzles, a system is achievedin which the sum of the k-values is always substantially constant. Byusing a pressure medium to control the valves in the return line, thenumber of solenoid valves needed can be reduced. The pressure mediumused for the control of the return line valves and/or a second pressuremedium can be circulated to the nozzles to keep them clean when noliquid to be injected is passed through them.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the invention will be described in detail by the aidof an example with reference to the attached drawings, wherein

FIG. 1 presents a diagram representing an apparatus according to theinvention,

FIG. 2 illustrates the supply of a liquid quantity per unit of time as afunction of engine load in the apparatus of the invention,

FIG. 3 presents a second embodiment of the solution of the invention indiagrammatic form, and

FIG. 4 presents a third embodiment of the solution of the invention indiagrammatic form.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 presents a diagram representing the method and system of theinvention in diagrammatic form. The system comprises at least twonozzles 1 a, 1 b, 1 c, 1 d, which are disposed in the engine's airintake duct or in a corresponding space leading to the combustionchamber of the engine for humidification of the intake air. In the casepresented in the figure, four nozzles are shown, with a channel 2 a, 2b, 2 c, 2 d leading to each nozzle from a supply pipe 4 supplying apressure medium, preferably an aqueous liquid. The pressure medium isfed into the supply pipe by a pump 6, driven by a drive device 7. Thepump pumps the pressure medium from a pressure medium source 10, such astank. Reference numbers 8 and 9 indicate a pipe and a relief valvethrough which the liquid can flow in case the pump pressure and thepressure in pipe 8 exceed a certain presettable limit value. Referencenumbers 13 and 15 indicate valves, and reference number 14 indicates afilter. The filter prevents particles that could clog the valves 1 a, 1b, 1 c, 1 d of the spraying head from entering the spraying system. Whenthe liquid surface in the container 10 falls below a certain level, alevel switch 11 will open valve 13. Switch 24 will close the valve whenthe water level in the container 10 has risen to a given height.

The pump 6 is preferably a constant-output pump which always pumps thesame amount Q of pressure medium per unit of time into the supply pipe 4when running. The pump drive 7 is preferably a motor, such as anelectrically operated direct-current motor, which drives the pump at aconstant speed. The channels 2 a, 2 b, 2 c, 2 d leading to the nozzlesare provided with valve elements A1, B1, C1, D1, which can be opened andclosed as instructed by the control system. The control system typicallycontrols the valves A1, B1, C1, D1 according to the required amount ofliquid to be sprayed, preferably according to the engine load, so theamount of liquid supplied into the intake air typically increases withthe engine load. The system comprises a return pipe 5, through which theliquid quantity not fed into the intake air returns to the tank 10.Disposed between the supply pipe 4 and the return pipe 5 are valveelements A2, B2, C2, D2, which can be opened and closed as instructed bythe control system. For each closed feed channel 2 a, 2 b, 2 c, 2 dleading to the nozzles 1 a, 1 b, 1 c, 1 d, a corresponding channel 3 a,3 b, 3 c, 3 d opening into the return pipe 5 is provided. If all thenozzle feed channel valves A1, B1, C1, D1 are open, then the valves A2,B2, C2, D2 in the flow passages leading to the return pipe 5 are closed,and vice versa. The sum of the k-values of the return channelssubstantially corresponds to the sum of the k-values of the closednozzles and those of their feed channels. In the embodiment representedby the figure, each channel 3 a, 3 b, 3 c, 3 d leading into the returnpipe 5 is provided with a throttle element, which is adjusted to matchthe k-value of the nozzle in closed state. Thus, the sum of the k-valuesin the system remains substantially constant. In the case of FIG. 1,valve element A1 in the feed channel 2 a leading from the supply pipe tovalve 1 a is open, thus allowing the liquid to flow to the nozzle. Thevalves B1, C1, D1 in the feed channels leading to the other valves areclosed, thus preventing liquid flow to valves 1 b, 1 c, 1 c.Correspondingly, valve A2 in the channel 3 a leading to the return pipe5 is closed, preventing liquid flow through channel 3 a into the returnpipe. Valves B2, C2, D2 in the other channels 3 b, 3 c, 3 d arrangedbetween the supply pipe and the return pipe are open, permitting theliquid to flow through them into the return pipe 5. The channels areprovided with a throttling 17 b, 17 c, 17 d or equivalent, whichcorresponds to the k-values of the closed nozzles. By providing nozzleshaving different characteristics and different flow rate capacities, avery large control range can be covered accurately. In the case of FIG.1, by using a pump with an output capacity of 15 l/min, where nozzle 1 ahas an output of 1 l/min, nozzle 1 b an output of 2 l/min, nozzle 1 c anoutput of 4 l/min and nozzle 1 d an output of 8 l/min, the entire rangeof 1-15 l/min can be covered by opening and closing the valves. Thepressure is typically constant in the system. When the engine loadincreases, the amount of liquid flowing into the intake air through thenozzles is increased by increasing the number of nozzles and/or byselecting a nozzle that permits a larger liquid quantity to flow throughit in a unit of time. When the engine load decreases, the amount ofliquid flowing through the nozzles supplying liquid into the intake airis reduced by reducing the number of nozzles and/or by selecting anozzle that permits a smaller amount of liquid to flow through it in aunit of time. In connection with the above-described operation, theamount of water supplied into the return pipe by the “by-pass” route iscorrespondingly adjusted in inverse proportion to the amount of waterfed through the nozzles. In a corresponding manner, the throttling isadjusted so that at least when liquid is being injected into the intakeair in the system, the sum of the k-values (Σk) remains substantiallyconstant regardless of whether the liquid is passed through the nozzlesor through the return pipe or whether a portion of the liquid quantityis passed through the nozzles and another portion, substantially therest of it through the return pipe. The flow rate for a nozzle is givenby the formula Q=k√p, where Q is the flow rate, p is the pressureforcing the medium through the nozzle and k is the nozzle resistance.The value of the factor k depends on the area of the nozzle aperture,among other things. In the case of circular aperture, the value of thefactor k depends on the aperture diameter d according to the equationk=0.78*d² when the aperture is a so-called short aperture. Theresistance of the return pipe is adapted to correspond to the resistanceof the closed nozzles.

FIG. 3 presents a second embodiment of the apparatus of the invention.In this case, a second pressure medium source 20, e.g. pump unit forpumping pressurized air is provided in connection with the sprayingapparatus. The second pressure medium is supplied through a supply pipe21 into the channel 2 a, 2 b, 2 c, 2 d leading to the nozzles 1 a, 1 b,1 c, 1 d via second feed channels 25 a, 25 b, 25 c, 25 d. The feedchannels 25 a, 25 b, 25 c, 25 d are connected to the channels leading tothe nozzles at a point between valve elements A1, B1, C1, D1 and thenozzles 1 a, 1 b, 1 c, 1 d. The second valves A2, B2, C2, D2 arranged inconnection with the channels leading to the return line 5 are controlledusing the pressure of the first pressure medium. When the control systemgives a signal and at least one of the first valve elements A1, B1, C1,D1 is opened, the first pressure medium, typically water to be injectedcan enter into the second feed channel 25 a, 25 b, 25 c, 25 d arrangedbetween the valves and the nozzles. The pressure medium can act on theregulator 24 a, 24 b, 24 c, 24 d controlling the second valve elementA2, B2, C2, D2, this pressure being greater than the spring load of theregulator, with the result that at least one of the valve elements isclosed. A second pressure medium, such as a liquid or gas or a mixtureof these, is supplied through the feed channels 25 a, 25 b, 25 c, 25 dto the nozzles not in use. The pressure of the second pressure medium istypically lower than the pressure of the first pressure medium in thesupply pipe 4. Therefore, when first valve element is opened, the firstpressure medium can enter the second supply pipe regardless of thepressure of the second pressure medium acting there. Each one of thesecond feed channels 25 a, 25 b, 25 c, 25 d is provided with a checkvalve 23 to prevent the first pressure medium from entering the secondsupply pipe 21. After the nozzle has stopped spraying, the system worksin the converse manner. Thus, when the first valve element is closed,the pressure in the second supply channel falls, with the result thatthe spring force in the regulator of the second valve exceeds thecounter-force of the pressure medium and opens the second valve element.The second pressure medium is now able to flow into the second feedchannel, forcing the first pressure medium remaining there to flowbefore it into the nozzle. The second pressure medium is allowed to flowthrough the nozzle, thus preventing the nozzles from becoming clogged inthe air intake duct. The pressure of the second pressure medium in thepiping is e.g. 6 bar. The force produced by the spring element of theregulator of the second valve element corresponds to a pressure of e.g.10 bar, so the second valve element will close when the pressure in thesecond feed channel exceeds 10 bar. Naturally, instead of a springelement, other elements capable of corresponding operation may also beused.

A method for controlling a spraying apparatus, especially a sprayingapparatus designed for the humidification of intake air, said apparatuscomprising at least two spraying nozzles 1 a, 1 b, 1 c, 1 d forinjecting liquid into the intake air. In accordance with the controlsystem's instructions, when the required amount of liquid to be suppliedthrough the nozzles increases, liquid flow passages are opened for morenozzles 1 a, 1 b, 1 c, 1 d and/or the liquid flow is directed into aflow channel leading to a nozzle that permits a larger liquid flowthrough it per unit of time, and when the required amount of liquid tobe supplied through the nozzles decreases, liquid flow channels areclosed at least for some of the nozzles and/or the liquid flow isdirected to a nozzle that permits a smaller liquid flow through it perunit of time. A substantially constant amount of liquid per unit of timeis fed into the supply pipe 4 and at least a portion of the liquidquantity supplied that is not directed to the nozzles is conveyed intothe return pipe 5. The pressure in the supply pipe 4 is keptsubstantially constant, regardless of the number of spraying headnozzles. The k-value (resistance) of at least one of the channelsleading into the return pipe 5 is adjusted to make it correspond to thek-value (resistance) of the closed nozzles. The sum of the activated(open) k-values is maintained at a substantially constant value. Theliquid to be fed into the return line is conveyed through at least onereturn channel 3 a, 3 b, 3 c, 3 d into the return pipe. The returnchannel 3 a, 3 b, 3 c, 3 d is provided with at least one valve elementA2, B2, C2, D2, which is controlled on the basis of impulses given bythe control system. In at least one return channel 3 a, 3 b, 3 c, 3 d, athrottle element 17 a, 17 b, 17 c, 17 d or equivalent having a k-valueadapted to correspond to the k-value of the at least one closed nozzlesis used. The amount of liquid to be supplied through the nozzles 1 a, 1b, 1 c, 1 d is adjusted as a function of the engine load. Through thenozzles 1 a, 1 b, 1 c, 1 d, a liquid mist, especially a water mist isInjected. The liquid mist is injected at a pressure of 10-300 bar. Themaximum droplet size of the liquid mist injected is typically 200micrometers. A second medium is conveyed to the nozzle when in it is inclosed state to prevent clogging of the nozzle.

An apparatus for supplying a liquid mist into the intake air of anengine, said apparatus comprising at least two nozzles for injecting aliquid mist into the air intake duct. The apparatus comprises at leasttwo feed channels 2 a, 2 b, 2 c, 2 d provided with valve elements A1,B1, C1, D1, a control system, which gives impulses on the basis of whichthe aforesaid valve elements are opened and closed, liquid supply means4, 6, 10 for supplying an aqueous liquid into a feed channel 2 a, 2 b, 2c, 2 d leading to at least one nozzle. The apparatus further comprisesan outlet pipe 5 and at least one outlet channel 3 a, 3 b, 3 c, 3 d,through which a connection to the outlet pipe from the supply pipe 4leading to the nozzles can be opened and closed. The outlet channel 3 a,3 b, 3 c, 3 d is provided with a valve element A2, B2, C2, D2, which hasbeen arranged to close when the corresponding valve element A1, B1, C1,D1 in the feed channel 2 a, 2 b, 2 c, 2 d leading to the nozzle opensand to open when the valve element in the corresponding feed channelleading to the nozzle closes. The apparatus comprises means for keepingthe flow resistance (sum of k-values) constant. The apparatus comprisesa number of feed channels 2 a, 2 b, 2 c, 2 d leading to the nozzles anda corresponding number of return channels 3 a, 3 b, 3 c, 3 d as well asvalve elements for each feed channel and return channel, each feedchannel-return channel pair being controlled together so that when thefeed channel opens, the return channel closes and vice versa. The returnchannels are provided with a throttle element 3 a′, 3 b′, 3 c′, 3 d′ orequivalent. The liquid supply means comprise a liquid source 10 and apump 6. The control system has been adapted to control the apparatus onthe basis of engine load. The apparatus comprises means 20, 21, 25 a, 25b, 25 c, 25 d for conveying a second pressure medium to a nozzles 1 a, 1b, 1 c, 1 d whose feed channel is in closed state to prevent clogging ofthe nozzle.

At least one spraying head 1 a, 1 b, 1 c, 1 d of the spraying apparatusis connected directly to the air intake duct structures, and a fine mistproduced by the spraying head comprising at least one nozzle is produceddirectly into the intake air in the air intake duct. When the solutionof the invention is used, no extra chambers or other containers areneeded in the air intake ductwork. The nozzles feed water mist under ahigh pressure into the air intake duct. The pressure is typically over10 bar, preferably over 30 bar, most preferably over 50 bar. Thepressure may be typically between 10-300 bar. The water is In the formof a fine mist. Preferably 50% of the water volume (Dv50) is in the formof droplets having a size typically below 200 micrometers, preferablybelow 100 micrometers and more preferably below 50 micrometers. Underhigh load conditions, the droplet size may be larger.

The nozzles in the spraying head may have different properties, whichhave been adapted according to the placement of each nozzle. The form ofthe spraying head, the number of nozzles and their orientation may varydepending on the application. It is also possible to supply differentmediums to the nozzle, such as water and gas. The figure does not showthe nozzles in detail, but they may be replaceable depending on theapplication.

FIG. 4 presents yet another solution according to the invention. Itcomprises nozzles 1 a, 1 b, 1 c, 1 d arranged in feed channels 2 a, 2 b,2 c, 2 d, each channel having a different number of nozzles placed atdifferent positions in the air intake duct K. In this embodiment, too,the valve elements A1-A2, B1-B2, C1-C2, D1-D2 controlling the liquidflow going into the nozzle feed channels 2 a, 2 b, 2 c, 2 d and thereturn channel 3 a, 3 b, 3 c, 3 d are controlled in pairs. These valveelement pairs are most appropriately controlled by means of solenoidvalves A1′, B1′, C1′, D1′. The return channels are provided withvariable throttles 17 a, 17 b, 17 c, 17 d, by means of which the flowcan be adjusted as desired. Correspondingly, the pressure can also bevaried by opening and closing the throttle elements in the returnchannel. In this embodiment, the valve elements and throttles arearranged as control blocks, indicated in the figure by the number 39 anda broken line. This embodiment likewise comprises a nozzle cleaningsystem, in which a pressure medium, such as pressurized air, is suppliedfrom a pressure medium source via a pipeline 21 by means of a pump. Thepressure medium supply line 21 of the cleaning system is provided with avariable throttle element for the control of the flow. The controlsystem further comprises a temperature regulating system, whereby thetemperature of the liquid to be injected can be adjusted. The systemcomprises a heat exchanger element 33 arranged in the return line 5, towhich heat can be supplied via a line and valve 38. When a small amountof liquid is to be injected, most of the liquid quantity supplied by thepump returns back via the return line. The pressure is at least partlyconverted to heat as it passes through the throttle elements 17 a-17 d,the liquid entering the return line being thus heated. From the returnline, at least some of the liquid can be conveyed directly to the pump 6or into the tank 10. In this case, the heat exchanger element 33 may besuperfluous because the system itself generates sufficient heat in theliquid. For example, in low-load conditions, if only 10% of the pumpoutput Is passed into the intake air, as much as 90% of the power of thepump motor will be transferred for heating the liquid. Similarly, theheat exchanger 33 may also recover heat and transfer it to another part.In the system, it is possible to obtain heat for the liquid to besprayed even without a heat exchanger. The return line 5 is alsopreferably provided with a filter element 34 for removing impuritiesfrom the liquid.

The nozzles are therefore of a type such that they produce a jet of finemist when fed with liquid under a high pressure. Many kinds of nozzlesof this category are known, e.g. from fire extinguishing technologyemploying water mist. For example, specifications WO 92/20454 and WO94/06567 disclose nozzles that produce a water mist at a high pressure.Naturally, other types of nozzles may also be used, e.g. specificationWO 01/45799 discloses yet another nozzle.

The amount of water supplied through the nozzles typically increaseswith increasing engine load. Thus, when the engine load is low, it ispossible to supply water only to some of the nozzles and increase thenumber of spraying nozzles when the load increases. Similarly, thespraying head can be provided with nozzles having different properties,such as flow rate, droplet size produced by the nozzles, etc. It is thuspossible to form different combinations, which can be adapted to a widerange of different applications, different engine types, differentplacements and conditions.

The apparatus of the invention is able to make full use of the quantityof heat required for the vaporization of the water, cooling the intakeair at each injection point to a temperature close to the wet bulbtemperature (or adiabatic saturation temperature, which in the case of awater-air mixture is practically the same thing), i.e. to thetemperature to which it is possible to reduce the air temperature byvaporization of water.

Thus, the humidity of the gas entering the cylinder and therefore theformation of nitrogen oxides is controlled within desired limits.

It is obvious to the person skilled in the art that the invention is notlimited to the embodiments described above, but that it may be variedwithin the scope of the claims presented below.

1. In a method controlling intake air humidification spraying apparatus,the apparatus comprising at least two spraying nozzles (1 a, 1 b, 1 c, 1d) for spraying a liquid into the intake air, the method improvementscharacterized in that, when an amount of the liquid to be suppliedthrough the spraying nozzles increases, liquid flow passages are openedfor more nozzles (1 a, 1 b, 1 c, 1 d) and/or the liquid flow is directedinto a flow passage leading to one of the spraying nozzles that permitsa larger liquid flow per unit of time than another of the sprayingnozzles, and when an amount of the liquid to be supplied through thenozzles decreases, liquid flow channels are closed at least for one ofthe spraying nozzles (1 a, 1 b, 1 c, 1 d) and/or the liquid flow isdirected to one of the spraying nozzles that permits a smaller liquidflow per unit of time than another of the spraying nozzles, and asubstantially constant amount of liquid per unit of time is fed into asupply pipe (4) for the spraying nozzles and at least a portion of theliquid quantity supplied that is not directed to the nozzles is conveyedinto a return pipe (5).
 2. Method according to claim 1, characterized inthat a constant pressure is maintained in the supply pipe (4),regardless of the number of the spraying nozzles spraying.
 3. Methodaccording to claim 1 characterized in that the k-value (resistance) ofat least one of the channels leading into the return pipe (5) isadjusted to make it correspond to the k-value (resistance) of thespraying nozzles in closed state.
 4. Method according to claim 1,characterized in that the value of the sum of the activated (open)k-values is kept substantially constant.
 5. Method according to claim 1,characterized in that the liquid to be fed into the return line isconveyed through at least one return channel (3 a, 3 b, 3 c, 3 d) intothe return pipe.
 6. Method according to claim 5, characterized in thatthe return channel (3 a, 3 b, 3 c, 3 d) is provided with at least onevalve element (A2, B2, C2, D2), which is controlled on the basis ofimpulses given by the control system.
 7. Method according to claim 5,characterized in that, in the at least one return channel (3 a, 3 b, 3c, 3 d), a throttle element (17 a, 17 b, 17 c, 17 d) or equivalenthaving a k-value adapted to correspond to the k-value of the at leastone closed one of the spraying nozzles is used.
 8. Method according toclaim 1, characterized in that the amount of liquid to be suppliedthrough the spraying nozzles (1 a, 1 b, 1 c, 1 d) is adjusted as afunction of the engine load.
 9. Method according to claim 1,characterized in that a liquid mist is sprayed through the sprayingnozzles (1 a, 1 b, 1 c, 1 d).
 10. Method according to claim 1,characterized in that the liquid mist is injected at a pressure of10-300 bar.
 11. Method according to claim 1, characterized in that themaximum droplet size of the liquid mist injected is typically 200micrometers.
 12. Method according to claim 1, characterized in that asecond medium is conveyed to the spraying nozzles when in it is inclosed state to prevent clogging of the spraying nozzle.
 13. Inapparatus supplying a liquid into intake air of an engine, theimprovements comprising: at least two spraying nozzles for spraying aliquid as a mist into a duct for the intake air; at least two feedchannels (2 a, 2 b, 2 c, 2 d) with valve elements (A1, B1, C1, D1)respectively leading to the spraying nozzles; a control system givingimpulses on the basis of which the valve elements are opened and closed;liquid supply means (4, 6, 10) for supplying the liquid into the feedchannels (2 a, 2 b, 2 c, 2 d); and an outlet pipe (5) and at least oneoutlet channel (3 a, 3 b, 3 c, 3 d), through which a connection to theoutlet pipe from the supply pipe (4) leading to the nozzles can beopened and closed.
 14. Apparatus according to claim 13, characterized inthat the outlet channel (3 a, 3 b, 3 c, 3 d) is provided with a valveelement (A2, B2, C2, D2) arranged to close when the corresponding valveelement (A1, B1, C1, D1) in the feed channel (2 a, 2 b, 2 c, 2 d)leading to the nozzle opens and to open when the valve element in thecorresponding feed channel leading to the nozzle closes.
 15. Apparatusaccording to claim 13, characterized in that the apparatus comprisesmeans for keeping the flow resistance (sum of k-values) constant. 16.Apparatus according to claim 13, characterized in that the apparatuscomprises a number of feed channels (2 a, 2 b, 2 c, 2 d) leading to thenozzles and a corresponding number of return channels (3 a, 3 b, 3 c, 3d) as well as valve elements for each feed channel and return channel,each feed channel—return channel pair being controlled together so thatwhen the feed channel opens, the return channel closes and vice versa.17. Apparatus according to claim 13, characterized in that the returnchannels are provided with a throttle element (3 a′, 3 b′, 3 c′, 3 d′)or equivalent.
 18. Apparatus according to claim 13, characterized inthat the liquid supply means comprise a liquid source (10) and a pump(6).
 19. Apparatus according to claim 13, characterized in that thecontrol system has been adapted to control the apparatus on the basis ofengine load.
 20. Apparatus according to claim 13, characterized in thatthe apparatus comprises means (33) for regulating the temperature of thefirst pressure medium.
 21. Apparatus according to claim 13,characterized in that the apparatus comprises means (20, 21, 25 a, 25 b,25 c, 25 d) for conveying a second pressure medium to a nozzle (1 a, 1b, 1 c, 1 d) whose feed channel is in closed state, to prevent cloggingof the nozzle.
 22. Apparatus according to claim 13, characterized inthat it comprises means (34, 36, 14) for filtering at least the firstpressure medium.
 23. In apparatus supplying a liquid into intake air ofan engine, the improvements comprising: at least two spraying nozzlesfor spraying a liquid as a mist into a duct for the intake air; at leasttwo feed channels (2 a, 2 b, 2 c, 2 d) with valve elements (A1, B1, C1,D1) respectively leading to the spraying nozzles; a control systemgiving impulses on the basis of which the valve elements are opened andclosed; liquid supply means (4, 6, 10) for supplying the liquid into thefeed channels (2 a, 2 b, 2 c, 2 d); and an outlet channel (3 a, 3 b, 3c, 3 d) is provided with a valve element (A2, B2, C2, D2) arranged toclose when the corresponding valve element (A1, B1, C1, D1) in the feedchannel (2 a, 2 b, 2 c, 2 d) leading to the nozzle opens and to openwhen the valve element in the corresponding feed channel leading to thenozzle closes.
 24. Apparatus according to claim 23, characterized inthat the apparatus comprises means for keeping the flow resistance (sumof k-values) constant.
 25. Apparatus according to claim 23,characterized in that the apparatus comprises a number of feed channels(2 a, 2 b, 2 c, 2 d) leading to the nozzles and a corresponding numberof return channels (3 a, 3 b, 3 c, 3 d) as well as valve elements foreach feed channel and return channel, each feed channel—return channelpair being controlled together so that when the feed channel opens, thereturn channel closes and vice versa.
 26. Apparatus according to claim23, characterized in that the return channels are provided with athrottle element (3 a′, 3 b′, 3 c′, 3 d′) or equivalent.
 27. Apparatusaccording to claim 23, characterized in that the liquid supply meanscomprise a liquid source (10) and a pump (6).
 28. Apparatus according toclaim 23, characterized in that the control system has been adapted tocontrol the apparatus on the basis of engine load.
 29. Apparatusaccording to claim 23, characterized in that the apparatus comprisesmeans (33) for regulating the temperature of the first pressure medium.30. Apparatus according to claim 23, characterized in that the apparatuscomprises means (20, 21, 25 a, 25 b, 25 c, 25 d) for conveying a secondpressure medium to a nozzle (1 a, 1 b, 1 c, 1 d) whose feed channel isin closed state, to prevent clogging of the nozzle.
 31. Apparatusaccording to claim 23, characterized in that it comprises means (34, 36,14) for filtering at least the first pressure medium.